This invention relates to a strip frame recognition apparatus which is installed in a communications station of a token ring local area network (LAN), and that recognizes communication data (hereinafter called "a frame") once transmitted to a ring from, and received by, the station as a frame to be stripped in order to prevent the frame from going around the ring more than one round.
In a token ring LAN, as illustrated in FIG. 1, all communications stations 1.sub.1, 1.sub.2 and 1.sub.3 are connected with one another in the form of ring and transfer a transmission right known as a "token" in the order of connection to the ring so that only one station among them can transmit at any time. A station which has transmitted a frame, when the station received the same frame, shall strip the transmitted frame in order to prevent the frame from going around ring 2 more than one round. In this case, judgement whether to strip a frame or not shall be based on the value of source address field (SA) included in the frame. Therefore, a conventional station sets the address of the station in its frame receiving circuit prior to starting operation and, when the SA of the received frame becomes equal to the set address, the station judges that the received frame is the frame to be stripped (hereinafter called "the strip frame"). In addition, the above-mentioned set address is used for judging whether the received frame is addressed to the station or not by coincidence of the set address with the value of destination address (DA) field of the received frame. Therefore, the set address is called a receive address.
Furthermore, typical structure of a frame is illustrated in FIG. 2. Namely, a frame consists of fields for each data, such as a start delimiter (SD) indicating the start of the frame, access control (AC) indicating the priority order of the frame, frame control (FC) indicating the classification of the frame, destination address (DA) indicating a destination station, source address (SA) indicating a source station, routing information (RI) indicating the route from the source station to the destination station, frame check sequence (FCS) to detect errors occurring in net communications information and communication end delimiter (ED) indicating the end of the frame, and frame status (FS) indicating receiving condition at the destination station. In addition, 48 bits or 16 bits is specified to be used for the SA field (16 bit address is used only in the configuration of smaller network; the case of 48 bits is mainly explained herein) and 32 bits for the FCS field.
In the LAN of token ring system, plural rings are connected with one another to form a more complicated network. Stations to connect these rings mutually are specifically called bridge stations.
In the network illustrated in FIG. 3, for example, two rings, 2.sub.1 and 2.sub.2, which have three stations respectively, are connected through bridge stations, 1.sub.B1 and 1.sub.B2. In this network, station 1.sub.1 belonging to ring 2.sub.1, addresses station 1.sub.3 belonging to ring 2.sub.2 (hereinafter reference numbers are used as addresses) using a frame set up with the information of DA=1.sub.3, SA=1.sub.1, RI=2.sub.1 to 2.sub.2. Bridge station 1.sub.B1 belonging to ring 2.sub.1, when receiving this frame, detects a requirement to transfer the frame to ring 2.sub.2 according to RI information, so 1.sub.B1 transfers the frame to bridge station 1.sub.B2 belonging to ring 2.sub.2. Bridge station 1.sub.B2 sends this frame to ring 2.sub.2 after obtaining the token of ring 2.sub.2.
In this case, since the content of the frame which bridge station 1.sub.B2 sents remains DA=1.sub.3, SA=1.sub.1, RI=2.sub.1 to 2.sub.2, bridge station 1.sub.B2 should set in its frame receiving circuit the address "1.sub.1 " of source station 1.sub.1, but not that of bridge station 1.sub.B2 in order to strip this frame.
As above, in the token ring LAN of FIG. 1, each station sets the receive address to its own individual address in its frame receiving circuit and recognizes the frame to be stripped and addressed to the station on the basis of the set value. However, in the LAN in which plural rings are connected with one another, when the bridge station transmits a frame received from another ring, the bridge should set in the frame receiving circuit the address of another station that is not the address of the bridge station. Consequently, the following problems will happen.
In the case as above, the bridge station resets its address in the frame receiving circuit when stripping of the frame received from another ring is completed. However, if an error happens in the network and the frame to be stripped is lost because of the error, the resetting time causes a long delay since the frame is not stripped. Therefore, even if the a new frame addressed to the bridge station is sent to the network before resetting the receive address, the bridge station cannot recognize the new frame as addressed to the station.
Furthermore, if an error condition happens, each individual address should be set in the receiving circuit of each station to recover the error. However, errors cannot be recovered from immediately because resetting of the station address at the bridge causes delays.
In order to solve these problems, there is an idea of an system in which a memory circuit of exclusive use for an address (hereinafter called strip address) is used for recognizing the strip frame and provided in a frame receiving part of the bridge station. Before transmission of a frame the SA of the frame is set in the memory circuit as the strip address by a control processor of the bridge station according to software. However, this system has the following problems to solve.
1) The bridge station should set a new strip address in the memory circuit every time before transmission of the frame, but time delay for transmission of the frame occurs because of time needed for software to execute a procedure of setting the strip address.
2) Program coding for the bridge, station becomes complicated and the quantity of the programming increases.
3) Since operation of a non-bridge station and that of a bridge station are different, it is difficult to realize both operations with the same hardware by use of an LSI having a built-in program.
In addition, such a system has a weak point that it needs a large scale circuit to inspect coincidence between the SA value of a memory circuit and SA of a received frame because the SA is as long as 48 bits.